The present invention relates to displaying waveforms retrieved from implantable medical devices. More particularly, this invention relates to storing and annotating portions of such a waveform.
Implantable medical devices are becoming increasingly more complex. As these devices become more complex they perform more functions. These devices frequently measure some physiologic parameter such as a patient's intracardiac electrogram, blood oxygen level, activity level, respiration rate, or body temperature. These devices may also analyze the measured parameter. Some devices provide some response as a result of that analysis. Often, the analysis may have multiple levels of detail associated with it. There may be a simplistic instantaneous diagnosis of the patient's condition, as well as a longer term, more sophisticated analysis. For example, a cardiac monitoring device may detect a series of contractions in the patient's right atrium. A more sophisticated analysis of this same signal would include rate information, and the detection of a possible atrial tachycardia or atrial fibrillation if the rate of the detected atrial contractions exceeds a preset rate for some preset length of time. Other implantable medical devices that could utilize similar diagnostic capabilities include, muscle stimulators, drug pumps, nerve stimulators, artificial organs and/or cardiac stimulating devices.
Cardiac stimulating devices such as pacemakers and cardioverter/defibrillators measure the patient's intracardiac electrogram or electrocardiogram. Newer devices may also have the capability to react to various physiologic parameters such as a patient's activity level or blood oxygen level. Cardiac stimulating devices may also be capable of terminating potentially dangerous cardiac arrhythmias. To perform these tasks, cardiac stimulating devices contain microprocessors and control circuitry that allow the devices to respond to a patient's heartbeat and other measured parameters. The patient's internal heartbeat signals, which are known as the intracardiac electrogram, are measured through leads attached to the patient's heart.
A cardiac stimulating device can respond to different cardiac conditions with varying levels of therapy. For example, some cardiac stimulating devices have the capability to treat heart arrhythmias such as tachycardia. Such devices confirm the onset of tachycardia when a set of detection criteria has been satisfied. Once a tachycardia episode is confirmed, increasing levels of therapy can be applied to the patient's heart to attempt to terminate the episode.
Many cardiac stimulating devices have adjustable settings, which a physician can select and enter into the device using a "programmer." Although default settings for the detection criteria and corresponding levels of therapy can be used, it is usually beneficial if the physician programs each individual patient's cardiac stimulating device, so that the specific characteristics of the patient's heart can be taken into account. Without tailoring the device settings, some patients may experience discomfort if they suffer from a cardiac condition that is not detected and treated adequately or sufficiently early by the cardiac stimulating device.
In order to properly adjust the settings of a cardiac stimulating device, physicians often analyze a patient's electrocardiogram (EKG). Physicians also analyze a patient's intracardiac electrogram, because the intracardiac electrogram is the signal that the cardiac stimulating device actually monitors during operation. The cardiac simulating device transmits the intracardiac electrogram to the programmer in real-time using radio-frequency telemetry. Either the intracardiac electrogram or the EKG may be displayed on a monitor or a chart recorder, for the physician's review.
During the course of monitoring the patient's heart, the cardiac stimulating device must identify which portions of the intracardiac electrogram represent certain cardiac events, such as the occurrence of a P-wave or an R-wave. Event markers, such as the letter "P" representing a measured P-wave, are generated whenever a cardiac event is detected by the cardiac stimulating device. Additional markers corresponding to events such as the application of a pacing pulse to the ventricle of the patient's heart are also generated. These markers are also transmitted to the programmer.
As described in Snell et al. U.S. Pat. Nos. 4,596,225 and 4,791,936, when displaying the intracardiac electrogram, it is a useful aid to display the event markers on the monitor or chart recorder adjacent to the event that each marker represents. For example, as shown in the aforementioned Snell et al. patents, the letter "P" can be placed on the display above the portion of the intracardiac electrogram that represents a P-wave. The letter "A" can be placed on the display above the portion of the intracardiac electrogram that represents application of a pulse to the atrium. Other events, such as the occurrence of an R-wave, or the application of a pulse to the ventricle can also be labeled "R" and "V", respectively. When the intracardiac electrogram is labeled with the event markers, the physician can more easily determine if the cardiac stimulating device is operating properly.
However, the response of the cardiac stimulating device to the measured intracardiac electrogram may be quite complex. In many cases, the physician may not be able to quickly determine if a device is responding correctly, because the cause of the device's behavior is not readily apparent. For example, it may be unclear when a tachycardia episode has been confirmed or what level of therapy is being applied to a patient's heart. Another example of a situation in which the behavior of the cardiac stimulating is unclear occurs when the cardiac stimulating device drops a pulse in order to terminate an episode of pacemaker mediated tachycardia ("PMT"). A PMT episode may occur when a pulse that is applied to a patient's ventricle is fed back to the patient's atrium via a retrograde conduction path in the heart. If the cardiac stimulating device is tracking the patient's atrial heart rate, the pulse that is fed back may be mistaken for the patient's normal sinus rhythm, which causes the cardiac stimulating device to apply increasingly fast pacing pulses to the patient's ventricle, until the maximum allowed pacing rate is reached. A pulse can be dropped to attempt to break this potentially harmful condition after a number of pulses have been applied at the maximum rate. However, unless the physician is aware that this is the reason for the missing pulse, it may appear that the device is not functioning properly.
The physician may not immediately know whether it is appropriate for a cardiac stimulating device to drop a pulse because the device makes this determination over a period of many heartbeats. For example, the cardiac stimulating device can be programmed to drop a pulse if more than 127 pulses are applied at the maximum rate. The device is allowed to apply a relatively large number of pulses to the heart at the maximum rate, because it may cause discomfort to the patient to drop pulses more frequently. However, it may be impractical for the physician to review the rate at which the last 127 pulses were applied. This information might no longer appear on the display, or even if the pulses were recorded in memory or on a strip chart, manual review might be unduly burdensome. Additionally, the behavior of each cardiac stimulating device differs, which further impedes the physician's efforts to evaluate device performance. Devices from different manufacturers are often incompatible and the operational characteristics of many cardiac stimulating devices depend on the selected values of the programmable parameters.
It would therefore be desirable to annotate the intracardiac electrogram or electrocardiogram to more completely reflect the state of the cardiac stimulating device. For example, when the cardiac stimulating device drops a pulse to terminate a PMT episode, the cardiac waveform adjacent to the missing pulse could be labeled "attempted PMT termination." When a cardiac stimulating device applies pulses to the heart in response to the confirmation of an episode of tachycardia, the annotation "tachycardia confirmed" could appear.
Alternatively, it may be desirable to annotate the intracardiac electrogram or electrocardiogram to more completely reflect the state of a cardiac monitoring device. In another alternative, any implantable medical device that monitors a measured value may have additional annotations displayed when accumulated individual measurements trigger detection of a more complex state by an implantable medical device.